Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disorder causing muscle weakness, followed by loss of motor function, leading to death. ALS patients typically live just three to five years following the first manifestation of symptoms. In a clinical setting, ALS is distinguished from other motor neuron diseases on the basis of initial symptom presentation, but may in fact belong to a group of motor neuron diseases sharing some substantive aspects of their pathophysiology or even etiology.
The pathology of ALS is manifested by motor neuron degeneration and death. Cortical motor cells disappear leading to retrograde axonal loss and gliosis in the corticospinal tract with ultimate atrophy of the spinal cord. During disease progression the ventral roots thin, and there is a loss of large myelinated fibers in motor nerves. Findings sometimes include a loss of frontal or temporal cortical neurons, and this can manifest symptomatically as ALS with frontotemporal dementia (ALS-FTD) although the typical course of the disease leaves cognitive functions intact. Intracellular inclusions in degenerating neurons and glia are common pathological findings of ALS; these include microfilament inclusions in spinal motor neurons, which may be associated with immunoreactive SOD1 particularly, but not exclusively, in familial ALS (FALS).
In Europe and North America, the incidence of ALS is about 2 cases per year per 100,000, while prevalence is around 5 cases per 100,000 with approximately 7,000 cases diagnosed annually in the US. Incidence increases with age, especially after 40, to a peak in the mid 70's. While age is a principle risk factor, family history is also important, with about 1 in 10 cases of ALS being familial. A number of environmental and behavioral risk factors, including smoking, have been proposed but none substantiated. Genetic analyses of sporadic ALS cases (SALS) have demonstrated significant associations between SALS and a range of loci. Mutations in loci more typically associated with FALS (superoxide dismutase 1 and others) have also been detected in SALS cases.
The majority of FALS cases follow an autosomal dominant inheritance pattern. About 20% of FALS patients are classified as Type 1 FALS, where the neurodegenerative phenotype is associated with inherited mutations to the superoxide dismutase type 1 (SOD1) gene localized in chromosome 21q22. Over 140 mutations of the SOD1 gene have been reported, almost all associated with ALS, but with significant variation in penetrance, age of onset, as well as nature and progression of symptoms. Some inherited SOD1 mutants with particularly weak penetrance and presumably, de novo somatic SOD1 mutants, may present as idiopathic (sporadic) ALS. It is also possible that a range of other genetic, physiological or environmental factors, isolated or in combination, may deleteriously impact the production and assembly of normal SOD1 enzyme, even in a genotypic background of wild type SOD1, and that this may in turn be linked to at least some further cases of idiopathic ALS.
Though the etiology of ALS is unclear, numerous mechanisms have been proposed, including SOD1-mediated toxicity. SOD1 is a copper and zinc containing metalloenzyme serving to deactivate superoxide radicals. It is ubiquitously expressed and is one of the most common intracellular enzymes, suggesting the function of the enzyme is important to the mediation of oxidative damage to cells. However, the loss of SOD1 function is not sufficient to account for the narrow and specific pathogenesis of the disease and it is generally thought, consistent with the dominant inheritance pattern, that it is some positive property of the mutant SOD1 that is implicated rather than a lack of function in the mutants.
One of the proposed mechanisms of mutant SOD1 toxicity is misfolding of the protein. Most SOD1 mutants fail to stably incorporate one or more metal ions and consequently fail to assemble as wild-type SOD1. The misfolded protein typically exposes reactive residues that are internal to the enzyme in wild-type SOD1, and perhaps as a consequence, SOD1 appears to form aggregates in ALS. These aggregates could be toxic themselves, or a secondary effect of the misfolding that is not the primary toxic mechanism.
There is evidence that mutant SOD1 expression is related to disease progression. Mutant SOD1 expression in non-neuronal cells may also be involved in the pathogenesis of ALS, while other evidence indicates that some properties of spinal cord tissue predispose cells to a pathological pathway that might damage other tissues subsequently, were a patient to survive the neurodegenerative disease. However, it is not clear what serves to initiate disease, and most type 1 FALS patients carry and presumably express mutant SOD1 for decades prior to the onset of the symptoms.
Several mouse and rat models expressing mutant forms of SOD1 exist. The experimentally induced mutations G93A, G37R, and G85R in the transgenic mouse models have phenotypes similar to human ALS. While there are also naturally occurring mouse models, the transgenic SOD1 mouse is considered the most accurate representation of the disease process.
The treatment options for ALS are limited. Riluzole is the only drug to have any established impact on survival in ALS. Riluzole can slow ALS progression to a modest degree, but its precise mechanism of action in ALS is unclear and ultimately it offers neither a cure nor sustained remediation of the condition. Multiple drugs have been tested for effectiveness in treating ALS but all others have failed to show efficacy in human clinical trials.
There is a need for effective therapeutics to treat and ameliorate the symptoms of ALS and diagnostics to assist in the identification of patients suffering from ALS.